Industry produces many aqueous mixtures from which water must be removed for most effective disposal of its solid content. Sometimes this is accomplished by screening techniques, and indeed this system does utilize some elements that are frequently used in screening operations. However, its specific constructions and mode of operation distinguish it from common screening and thickening operations, and emphasis is therefore placed upon the removal of water or carrier fluid rather than on the separation of the solid as the ultimate objective.
The phrase "aqueous mixture" is utilized to include suspensions, coarse mixtures, colloidal solutions and combinations of these. Their phenomena and their interaction in liquid mediums is very much interdisciplinary in nature both from the point of view of the underlying fundamental principles, and the resultant effect of outside and induced influences upon the solids when they are dispersed in liquid mediums. The resultant phenomena are highly complex and depend upon many chemical, physical and biological factors, including particle size, shape, flexibility or lack of flexibility, the chemical and electrical properties of particle surfaces, interaction of particles with others of both like and unlike nature, and the interaction of the particles with the liquid medium. In addition to these are such effects as the kinetic and electrokinetic properties, and rheological or mechanical properties such as viscosity of a dispersion.
The primary forces involved in the transportation and adhesion of particles in a liquid medium are as follows:
1. Forces and torques relating to the motion of the fluid causing the Brownian motion of particles, which is to say hydrodynamic and diffusion effects.
2. External forces such as those due to electrical, magnetic and gravitational fields including those which are externally induced.
3. Chemical and colloidal forces which result from the interaction of substrate surfaces, particles and molecules in the suspending medium.
Analyses of the separation process have shown that the role of "surface interaction" on the transport of the particle from the bulk of the suspension medium in the vicinity of the substrate or screen surfaces is generally quite small.
In general, liquid suspensions passing along a side hill screen or sieve try to pass through the openings. In so doing they attempt to follow the shortest route as a consequence of gravitational pull. It requires kinetic energy to jump or bridge these screen openings, and such energy is quickly dissipated at the start of the screening surface. A liquid suspension passing through the openings screens out those particulates by size which are larger than the openings. These screened-out solids effectively "blind" or plug the openings as a continuing stream of liquid attempts to pass through it. At a given point there is insufficient surface area of free opening space, and the liquid suspension overrides the blinded area, passing down the surface until it finds an opening it can pass through. Eventually the entire open surface becomes plugged and the entire liquid suspension merely runs over and off the end of the screen.
The liquid suspensions in trying to pass through these restricted openings are subject to the Venturi effect which in turn increases the velocity of the solution as it attempts to make passage. Dependent upon size, shape and depth of openings this acceleration is by degree further accentuated by "Coenda" wall attachment phenomena.
The resultant Venturi effect is to drive and wedge oversize solids into said openings. This is particularly noticeable in the case of suspended/colloidal solids that are soft/semi soft or glutinous in nature. (Example--organic particles vis-a-vis inorganics such as sand.)
Prior screens and sieves rely upon the dissipation of turbulence and kinetic energy within the headbox assembly with a view to establishing smooth laminar flow over a weir or via nozzle(s) onto a flow stabilization initial (non perforate) surface.
The effect (and as claimed by others) creates a continuous smooth laminar sheet of liquid, which by its very nature tends to align suspended solids parallel to the flow stream, before meeting the screening surface.
The result claimed is that the greater length (as in stringy fibrous solids) is presented to screen openings perpendicular to the aperture or slot, thereby allowing for capture of solids which might otherwise (by nature of smaller side--width vs. length) pass through said openings.
Further it is claimed that the smooth laminar sheet of liquid mixture is sheared or peeled off layer by layer as it passes on down the screening surface, until only captured solids exit at the bottom of the sieve.
Each claimant's and/or manufacturer's device has its own design variations, but all utilize the continuous flow, energy dissipating laminar stream for presenting a mixture to a screening surface.
Extensive operating studies by the inventor with a broad range of these devices, has provided insight into why they "plug" or "blind", a complaint which is common across a broad range of industrial and municipal installations. The continuous stabilized laminar flow once it reaches the screening surface, quickly dissipates the entrained kinetic energy in attempting to bridge or jump screen openings. Once this energy is dissipated (usually within the first nine to twelve inches of screening surface) the mixture is subject only to gravitational pull (ie. shortest route) and subsequently tries to pass the entire mixture through substrate openings. This is further accentuated as (noted earlier) by Venturi and wall attachment phenomena present. The Venturi effect imparts hydrodynamic and shear forces upon the mixture, which redefine shape, size and configuration of particulates contained therein. In the case of most soft and semi solids, the induced stresses cause them to break down and pass into solution as finite and dissolved solid. The net effect is that a high percentage of particulates of a size originally larger than the substrate openings, now pass through with carrier fluid! The remaining solids or screened particulates are subjected to the same forces which cause them to compress and wedge themselves within openings.
As the substrate openings are closed off with trapped solids, an interesting phenomena takes place--namely:
Once flow through an aperture is closed off, the stress and compressive forces are reduced, with the result that the trapped solids within substrate apertures or openings actually expand. In doing so they firmly wedge or lock themselves into said openings. The screen openings are now effectively plugged or `blinded`
This condition is maintained by the continuous flow stream passing over the blinded area creating an encapsulating blanket effect. At a given point there is insufficient surface area of free open space--wherein the liquid suspension overides the blinded area, pass on down the surface, until it can pass through the screen. Eventually the entire open surface becomes plugged, and liquid suspension just runs over and off the end of sieve.
The laminar effect acts much like a classifier, causing particulates and colloidal solids to entrain themselves by specific gravity, weight, size and shape in the various lamina layers.
It is noted that the various layers travel at different speeds, and are effectively isolated from one another. The upper layers travel at higher speeds, with each neighboring layer traveling slower down to the layer that is trying to carry suspended particulates through screen openings with it. The amount of layers is dependent upon thickness depth of liquid medium and the nature of suspended colloidal particulates (see opening paragraphs) and liquid medium.
In the theory the screened solids in the lowest layer next to substrate/screen, slough off and are passed on down the screen surface. The effect being (in theory) to concentrate the solids until they pass off the end of the sieve. In actuality, the layers of liquid passing over the bottom layer tend to bind or encapsulate the captured solids in `situ` as noted.
The claim that captured solids slough off through friction and reopen screen openings is unsubstantiated in practice. If sloughing does occur, it shears off only those solids above the screen surface, leaving the balance of the solids trapped within screen apertures, encapsulated and firmly in place. Further, said action creates a smooth imperforate surface that further accentuates the "run off" condition so prevolant.
The laminar layer next to the active surface zone, maintains the isolating blanket effect and thereby allows the corresponding layers above to flow on down unhindered.
Once all the open surface area is plugged and encapsulated the screen is effectively blinded--and represents to the liquid a highly lubricated chute or flume. At this point the liquid accelerates (less resistance) and runs right off the end of the sieve.
It has been the study and experience with such screen sieves over the years, that led to the understanding of laminar flow and the effects of same upon screening surfaces (side hill types).
The problems with "blinding, plugging" and "pin feathering" of existing screen types is best reflected in the numerous and often intricate methods manufacturers continue to employ in the attempt to alleviate them. Sprinklers, sprays, washers, and vibrators have all been tried with only limited success. Water sprays/washers force trapped solids through substrate apertures, thereby increasing the solids in filtrate or liquid.
Vibrators create the same effect mechanically, with the induced harmonic loosening the trapped solids so they can pass on through apertures with liquid. Further vibration of the substrate imparts forces which cause many colloidal/finite solids to go into a dissolved state, as mentioned earlier.
Along with the preceeding--and numerous other methods partial effectiveness can be claimed in the constant battle against "blinding", but they also inversely decrease the "solids capture" rate and proportionately reduce the device's efficiency in carrying out the task it was designed for!
An in depth study of this situation has led the inventor to conclude that a fundamental change in approach to dewatering is needed.